RRR.
I was never able to find a decent method of dialling in MVA so I developed my own many years ago, which follows.
That is a lot of cam for 225 cubes, but the long stroke helps to tame it at lower rpms where big cams are at their worst. I have never tried this on an engine with such low vac at idle, so I am unsure how it will work. The GM type adj VA units work down to about 5" of vac. Here is what I hope will happen: when you start the engine, you normally give the engine a little throttle. That slight rpm increase will pull a little more vac & hopefully pull in the VA plunger..& hold it in at idle speed.
First step is to find what I call the 'sweet spot', the most efficient idle timing. I used to use a digital tach for this, but no longer bother. I just do it by sound & feel.'
- disconnect the VA for now
- warm up the engine & put in gear if auto
- loosen dist clamp & slowly turn the dist to advance the timing. Keep going until you have the highest idle rpm. Toggle dist CW/CCW to make sure you have the highest rpm. Now check the timing. Say it is 40*. That is the 'sweet spot'. Your idle timing can be any combination of init + MVA. The adj VA units usually have a 30* range. With idle timing, a couple of degrees either way is not critical, unlike WOT timing.
- Adjusting the VA unit spring rate. It is imperative that the timing remains steady at idle. Often it varies because of misadjusted VA unit or factory unit used with a stiff spring. Then the comment, 'MVA didn't work for me' when the problem was not MVA...
- turn the AK fully CW to the softest spring setting. You will probably have to fabricate a stop for the VA arm to limit total travel. On the SB & BB Mopar dists, I silver solder the stop onto the arm, near the bend in the arm. Another method that I haven't tried might be a screw through the body that limits the travel of the plate.
- everything hooked up now, start the engine & check timing, in gear if auto. If it is steady, job done. To minimise light throttle ping, do this: turn AK 2 turns CCW & recheck timing; repeat this until timing drops off or becomes unsteady. Then go back & turn the AK 3 turns CW.
- if the timing is fluctuating, it is probably going to be because of the extremely low vacuum. I would prise the VA can apart & see if can find a softer spring that you can use & then repeat above.
Good luck!
